Method of rotary moulding a shell structure having an integral support structure

ABSTRACT

A method of forming a shell structure using rotary moulding techniques where two halves of the shell are moulded together and then joined when the two halves of the shell are in a molten state. The method is particularly useful for moulding one-piece boat hulls.

TECHNICAL FIELD

This invention relates to a moulded shell and method of forming thereof.

BACKGROUND ART

The invention may be particularly applicable to a moulded boat hullcomprising an outer shell and an inner shell.

One generally known method of forming a boat hull from moulded plasticshells it to mould separate outer and inner shells and then to join thetwo shells together either by using adhesives, or by fastening devicessuch as bolts, screws and the like. This method of construction enablesthe internal surfaces of the hull to be inspected for quality andthickness before assembly, and enables various shaped sections of thehull to be joined together where required to obtain the necessarystrength. In particular, it enables hollow girder sections to be formedlengthwise along the hull to provide rigidity of the hull. However withconventional methods of attachment of the outer and inner shells usingfastening devices there are inevitably stress concentrations in theregions of the joints, and in the case of using adhesives there is thepossibility of imperfect jointing also leading to local areas of stressconcentration. As a result the joints are susceptible to fatiguefailure. Consequently these methods of construction are not satisfactoryfor hulls of larger high powered vessels subjected to high operationalloadings over long periods of time. It is therefore desirable withmoulded hulls, to have an integral construction to facilitate even loaddistribution throughout the structure thereby minimising problems offailure at regions of attachment.

Another drawback with the conventional method of construction is that itinvolves two processes. The first process being to form the two shellsand the second process being to join the two shells together.Consequently the method is time consuming and hence costly.

An alternative method of construction that reduces production time andpartially overcomes the problem of joining is the rotary mouldingprocess. With the rotary moulding process a complete shell can bemoulded in a single step thereby speeding up production and reducinglabour costs. With this method, moulds for outer and inner shells of thehull, such as the hull bottom and stern, and the hull decking, seatingand flooring sections respectively are made so that they can be attachedtogether to form a single hollow mould.

In the process, a rotary plastic moulding powder such as Dupont 8405moulding powder of a predetermined amount is first poured into one ofthe shell moulds. Then after attaching the moulds together, theresulting hollow mould is heated while being rotated and tilted so thatthe powder can flow over all internal surfaces of the mould, sticking tothe internal surfaces as it melts under heat from the mould and forminginto a homogeneous plastic. By programming the sequence and degree ofrotation and tilting, the whole of the internal surfaces of the mouldcan be evenly covered with plastic to a predetermined thickness so thatan integral hollow shell is formed. With this method of construction,the peripheral edges of the moulded hull, that were necessarily bondedor fastened together in the conventional two process method, can beformed as homogeneous integral parts of the hull, thereby ensuring evenstress distribution in these regions to that fatigue failure isminimized. Furthermore certain other sections such as those aroundcentreboard wells where the bottom surface of the hull is not continuouscan also be formed as homogeneous integral parts of the hull connectingthe inner and outer shells of the hull together.

With the above rotary moulding technique however, due to the requirementof allowing the plastic to flow in powder form over all the surfaces tobe coated with plastic, it has not been possible to form internalsections of homogeneous plastic such as girder sections connecting theinner and outer shells where one of the surfaces of the shell iscontinuous.

It is an object of the present invention to provide a method of forminga shell structure that addresses the above problems and difficulties orat the very least offers a useful choice.

It is a further object of the present invention to provide a mouldedplastic shell formed by such a method.

DISCLOSURE OF INVENTION

According to one aspect of the present invention there is provided amethod of forming a shell structure using rotary moulding techniques asdescribed herein including the steps of:

(a) aligning oppositely disposed moulds in a spaced apart relationship,said moulds being shaped with complimentary peripheral edges and one orboth of said moulds being shaped to create a stiffening member in theshell structure formed on the mould or moulds,

(b) providing a peripheral member which covers a peripheral gap betweenperipheral edges of the spaced apart moulds,

(c) moulding a shell from molten plastic material on opposed surfaces ofthe moulds and the peripheral member, and

(d) advancing the moulds towards each other whilst the plastic materialis in the molten state so that contact is made between the moltenmaterial in the vicinity of the stiffening member to merge and unify theshell, whilst material at the peripheral edges is displaced with saidadvancement.

According to another aspect of the present invention there is provided amethod of forming a shell structure using rotary moulding techniques asaforesaid wherein said shell structure comprises a boat hull, saidoppositely disposed moulds comprise inner and outer shell moulds forsaid boat hull, and said inner mould is shaped to create at least onelongitudinal stiffening member for said boat hull, characterised by thesteps of:

(a) aligning said inner shell mould and said outer shell mould in aspaced apart relationship such that peripheral portions thereof are inalignment,

(b) providing said peripheral member around the periphery of said innerand outer shell moulds such that the peripheral gap therebetween iscovered, said peripheral member being arranged such that said oppositelydisposed moulds may be advanced towards each other over a predetermineddistance,

(c) moulding a boat hull from molten plastic material on opposedsurfaces of said inner and outer shell moulds and the peripheral member,

(d) advancing the inner and outer shell moulds towards each other whilstthe plastic material is in the molten state so that contact is madebetween the molten material on the inner and outer shell moulds in thevicinity of the stiffening member such that said molten material becomeshomogeneous in the region of contact, whilst material at the peripheraledges is displaced with said advancement,

(e) allowing said moulds to cool,

(f) separating said inner and outer shell moulds leaving said boat hullsupported in one or other of said moulds, and

(g) removing said boat hull from said mould.

According to another aspect of the present invention there is provided amoulded plastic shell structure formed by said rotary moulding techniquecomprising opposing shells which are integrally joined around peripheralportions thereof and which are also integrally joined at adjacentregions within said periphery at a region of continuity of an outersurface of one of the shells.

According to another aspect of the present invention there is provided amoulded plastic shell structure as aforesaid wherein said shellstructure comprises a boat hull and said opposing sections compriseinner and outer shells of said boat hull, wherein said inner shell isprovided with at least one longitudinal stiffening member which isformed integral with said outer shell at a region of continuity of anouter surface of said outer shell.

With the above method of forming a shell structure such as a boat hull,at least one longitudinal stiffening member may be integrally formedbetween inner and outer shells by simultaneously moulding combined innerand outer shells of the hull by a rotary moulding process and thenbringing the inner and outer shell moulds together so that adjacentsurfaces in the vicinity of the stiffening member are merged and unitedtogether.

In this process the inner and outer shells may be held apart by apredetermined amount to provide space for the moulding powder tocompletely flow over all surfaces including the surface forming thestiffening member. With the shells held apart in this condition theperipheral edges may be temporarily sealed to prevent the mouldingpowder from escaping from the moulds. A possible sealing means maycomprise peripheral plates fixed to peripheral edges of the inner andouter shell moulds so as to provide a seal to the moulding powder whileallowing the moulds to be moved together. Any gap between these platesmay be temporarily sealed with a high temperature sealant such assilicone. However any other suitable means may be used to provide a sealto maintain the powder or molten plastic within the assembled moulds.

With the shells sealed around their periphery, they may then be tiltedand rotated as one while being heated according to a predetermined cycleso that all internal surfaces are covered with molten plastic mouldingmaterial. The shells may then be brought together by a predeterminedamount so that moulding material at specified internal regions of theinner and outer shells, such as regions having stiffening members, maybe fused together to form a homogeneous connection.

In this way a moulded hull may be formed wherein predetermined internalregions may be homogeneously connected. This method has advantages overprevious methods of hull construction in that it enables stiffeningmembers to be located as required within the hull and homogeneouslyconnected to the hull shell, thereby reducing susceptibility to jointfailure inherent in bonded or fastened joints and the like. Furthermore,by using a rotary moulding process with a predetermined operation cycle,the hull can be produced quickly and uniformly thereby reducing costsand enhancing quality control.

BRIEF DESCRIPTION OF DRAWINGS

Aspects of the present invention will now be described by way of exampleonly with reference to the accompanying drawings in which:

FIG. 1 is a general layout of a boat hull according to a possibleembodiment of the present invention, and

FIG. 2 is a cross sectional view, corresponding to a half section atA--A of FIG. 1, showing inner and outer shell moulds for moulding theboat hull of FIG. 1, the inner shell mould and outer shell mould beingarranged in a separated condition for rotary moulding, and

FIG. 3 is a cross sectional view at A--A in FIG. 1 of a completed boathull formed by the moulds of FIG. 2 in accordance with a possibleembodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

With respect to FIG. 1 of the drawings there is provided a hull for apower boat generally indicated by arrow 1. The hull 1 is formed from amouldable plastic material and comprises an inner shell 2 and an outershell 3. The inner shell 2 is joined to the outer shell 3 along a jointline 4. The outer shell 3 comprises a keel 5 and planing strakes 6 and 7disposed between the keel 5 and joint line 4 and extendinglongitudinally from the region of the bow 8 to the region of the stern 9of the hull 1.

A hull such as that shown in FIG. 1 is formed by means of a shell mouldgenerally indicated by arrow 10 in FIG. 2. For convenience, only a halfsection of the mould 10 is shown as viewed at a location correspondingto section A--A in FIG. 1. The mould 10 comprises an inner shell mould11 and an outer shell mould 12.

The inner shell mould 11 is shaped so as to form an inner shell of aboat hull and comprises hull stiffening members generally indicated byarrow 13 extending along a longitudinal axis of the mould 10, thestiffening members 13 having lower faces 14, 15 and 16, upper flanges28, 29 and intermediate webs 30. Hence, the stiffening members provide aseries of internal supports with their lower faces 14, 15, 16 beingintegrally moulded to and extending inwardly from the interior surfaceof the outer hull 12, and having spaced intermediate webs 30, withfreely extending flanges 28, 29 at their opposite ends. An inner partingplate 17 with a wedge shaped protruding, rim or flange, 18 inclined toface outwards is provided substantially around the entire peripheraledge of the inner shell mould 11.

The outer shell mould 12 is shaped so as to form the outer shell 3 ofthe boat hull 1, with strake faces 19 and 20 for forming the planingstrakes 6 and 7 of FIG. 1, and keel face 21 for forming the keel 5 ofFIG. 1. The upper peripheral edge of the outer shell mould 12 is formedas a substantially planar mating surface 22. Around the peripheral edgeof the mating surface 22 is formed an outer parting plate 23 disposedsubstantially perpendicular to the mating surface 22.

The inner shell mould 11 and outer shell mould 12 are constructed suchthat when they are disposed adjacent to each other, the outermost edgeof the wedge shaped rim 18 of the inner parting plate 17 is in slidingcontact with an inner surface of the outer parting plate 23.

In FIG. 2 the inner shell 11 and outer shell 12 are shown separated fromeach other by means of a suitable separation mechanism (not shown in thefigure) such that a predetermined distance "d" is obtained between abottom edge of the wedge shaped rim 18 and the mating surface 22, andsuch that the strake faces 19 and 20 and the keel face 21 arerespectively separated from the lower faces 14, 15 and 16 of thestiffening members 13 by a distance "d+t" where "t" is the requiredfinal thickness of plastic in these regions. With the inner and outershell moulds 11 and 12 assembled in this condition, moulding powderwhich has been poured into one of the shell moulds before assembly, isfree to flow over the entire internal surface of the inner shell mould11 and the outer shell mould 12 as the assembled mould 10 is tilted androtated by a suitable mechanism (not shown in the drawings). To ensurethat the moulding powder does not leak out between the contacting edgesof the wedge shaped rim 18 and the outer parting plate 23, this area maybe sealed with a heat resistant flexible sealant 24 such as a siliconesealer.

The assembled mould 10 filled with a predetermined quantity of mouldingpowder is then heated by a suitable means, such as by a gas fired oveninto which the assembled mould 10 may be placed, to a predeterminedtemperature and is rotated and tilted according to a predeterminedprogram cycle. This ensures that the powder contained within theassembled mould 10 flows over the entire internal surface of the innershell mould 11 and outer shell mould 12 and melts to form a skin of apredetermined thickness.

On completion of the program cycle, the separation mechanism is operatedso that the inner shell mould 11 and the outer shell mould 12 may bebrought together by the before mentioned predetermined amount "d" sothat the bottom edge of the wedge shaped rim 18 abuts with the matingsurface 22. At this time, any flexible sealant 24 around the peripheraljoint is ruptured. During this process the mould plastic is still in asoft state. Consequently plastic formed on the surfaces of the innerface of the parting plate 23, the lower face of the wedge shaped rim 18and a portion of the mating face 22 is squeezed towards the interior ofthe mould so that a section along the joint between the inner shell 11and the outer shell 12 is formed to a predetermined thickness. This isshown as "w" in FIG. 3 which shows a half cross sectional view of thecompleted plastic hull view at A--A of FIG. 1.

Also at the time of bringing the inner shell 11 and outer shell 12together, adjacent internal surfaces of the plastic mould materialcovering the lower faces 14, 15 and 16 of the stiffening members 13 andthe strake faces 19 and 20 and the keel face 21 respective are broughttogether so that the plastic mould material is squeezed together in asoft state thereby forming a homogeneous connection as shown at strakeportions 25 and 26 and keel portion 27 in FIG. 3. The mould assembly isthen allowed to cool and harden, after which the inner shell mould 11 isseparated from the outer shell mould 12, leaving the completed hull 1accessible for removal from the outer shell mould 12.

I believe the advantages of my invention to be as follows, however itshould be appreciated that all such advantages may not be realized onall embodiments of the invention, and the following list if given by wayof example only as being indicative of potential advantages of thepresent invention. Furthermore, it is not intended that the advantagesbe restricted to those of the list which follows:

1. The method of forming moulded shells enables homogeneous internalconnections to be formed in shell structures such as internalconnections at stiffening members between inner and outer shells of boathulls, thereby reducing the likelihood of failure inherent inconnections involving bonded or fastened construction methods.

2. The method enables the rotary moulding process to be used for theproduction of large size boat hulls and similar heavily loadedstructures which require reliable connections between internalstiffening members, thereby enabling the advantages of the rotarymoulding process to be realized in the production of such large sizestructures.

3. By forming a hull of a boat as an integral unit with one or moreinternal stiffening members formed homogeneously with inner or outershells of the hull, a reliable hull structure can be achieved overcomingproblems of failure at joints inherent in conventional designs withbonded or mechanical jointing methods.

4. Forming the hull as an integral unit reduces construction timecompared to other methods where the process of moulding the hull isseparate from the process of connecting internal surfaces of the hulltogether by bonding or mechanical attachment methods.

5. The method enables internal hollow sections to be easily and reliablyformed inside a hull of a boat so that girder sections may be designedto provide sealed buoyancy compartments as well as to reinforce thehull.

Aspects of the present invention have been described by way of exampleonly and it will be appreciated that modifications and additions may bemade thereto without departing from the scope thereof as defined in theappended claims.

I claim:
 1. A method of forming a shell structure using rotary mouldingtechniques including the steps of:(a) aligning oppositely disposedmoulds having complementary opposed peripheral edges in a spaced apartrelationship, with a peripheral gap of predetermined distance d betweensaid opposed peripheral edges, said moulds being shaped with saidopposed peripheral edges being complementary, and at least one of saidmoulds being shaped to create a stiffening member in a shell structureformed on said moulds, (b) said stiffening member being formed atopposed mould surfaces which are initially spaced apart by a distanced+t, where t is a predetermined desired final thickness at a base ofsaid stiffening member, (c) providing a peripheral member which coverssaid peripheral gap between said peripheral edges of said spaced apartmoulds, (d) rotationally moulding a shell from plastic material in amolten state on said opposed surfaces of said moulds and said peripheralmember, and (e) advancing said moulds towards each other by saiddistance d whilst said plastic material is in said molten state so thatcontact is made between said plastic material in said molten state atsaid stiffening member to merge and unify said shell with a stiffeningmember of thickness t, whilst said plastic material in said molten stateat said peripheral edges is displaced from said peripheral gap to form asmooth peripheral joint of predetermined thickness with saidadvancement, thereby forming said shell structure.
 2. A method offorming a shell structure as in claim 1 wherein said shell structurecomprises a boat hull, said oppositely disposed moulds comprise innerand outer shell moulds for said boat hull, and said inner mould isshaped to create at least one longitudinally stiffening member for saidboat hull, comprising the steps of:(a) aligning said inner shell mouldand said outer shell mould in a spaced apart relationship such that saidperipheral edges thereof are in alignment and, spaced apart by said gapdistance d, (b) providing said peripheral member around a periphery ofsaid inner and outer shell moulds such that said peripheral gaptherebetween is covered, said peripheral member being arranged such thatsaid peripheral gap remains covered as said oppositely disposed mouldsare advanced towards each other over said predetermined distance d, (c)rotationally moulding a boat hull from said plastic material in saidmolten state on opposed surfaces of said inner and outer shell mouldsand said peripheral member, (d) advancing said inner and outer shellmoulds towards each other said distance d whilst said plastic materialis in said molten state so that contact is made between said plasticmaterial in said molten state on said inner and outer shell moulds atsaid stiffening member such that said plastic material in said moltenstate becomes homogeneous and is of said thickness t to form saidstiffening member following contact, whilst said plastic material insaid molten state at said peripheral edges is displaced with saidadvancement to form said smooth peripheral joint, (e) allowing saidmoulds to cool, (f) separating said inner and outer shell moulds leavingsaid boat hull supported in one or other of said moulds, and (g)removing said boat hull from said mould.
 3. A method of forming a boathull according to claim 2 wherein said peripheral member comprises innerand outer plates, said inner plate being attached to said inner shellmould and said outer plate being attached to said outer shell mould,said plates being arranged so as to slide relative to each other as saidinner and outer shell moulds are advanced towards each other.
 4. Amethod of forming a boat hull according to claim 3 wherein said methodfurther comprises the step of sealing between said inner and outerperipheral plates with a sealant prior to said step of moulding saidboat hull from said plastic material in said molten state.
 5. A methodof forming a boat hull according to claim 4 wherein said innerperipheral plate is formed with a shaped member which faces an innersurface of said outer peripheral plate.
 6. A method of forming a boathull according to claim 5, wherein said shaped member of said innerperipheral plate, said outer peripheral plate and said peripheral edgeof said outer mould, cooperatively squeeze said plastic material in saidmolten state at said opposed peripheral edges of said inner and outermoulds to form said smooth peripheral joint.
 7. A method of forming aboat hull according to claim 5, wherein said shaped member is in theform of an inclined edge which forms a peripheral flange.
 8. A method offorming a reinforced boat hull shell structure from oppositely disposedmoulds which comprise inner and outer shell moulds for said boat hull,and said outer shell mould including interior and exterior surfaces andsaid inner shell mould being shaped to create at least one longitudinalstiffening member for said boat hull, which stiffening member extendsbetween said inner shell mould and said interior surface of said outershell mould including the steps of:(a) aligning said inner shell mouldand said outer shell mould in a spaced apart relationship such thatperipheral portions thereof are in alignment and spaced apart by aperipheral gap of predetermined distance, (b) providing a peripheralmember around a periphery of said inner and outer shell moulds such thatsaid peripheral gap therebetween is covered, said peripheral memberbeing arranged such that said oppositely disposed moulds may be advancedtowards each other over said predetermined distance, (c) providing astiffening member forming surface on said inner shell mould and acomplementary receiving surface therefor on said interior surface ofsaid outer shell mould which is opposite said stiffening member formingsurface, (d) rotationally moulding a boat hull from plastic material ina molten state on said opposed surfaces of said inner and outer shellmoulds and said peripheral member, and (e) advancing said inner andouter shell moulds towards each other said predetermined distance whilstsaid plastic material is in said molten state so that contact is madebetween said plastic material in said molten state on said stiffeningmember forming surface of said inner shell mould and said complementaryreceiving surface on said interior surface of said outer shell mould,such that said plastic material in said molten state becomes homogeneousat said opposed contact surfaces to form said stiffening member betweensaid opposed contact surfaces, while said plastic material in saidmolten state at said periphery is displaced from said peripheral gap toform a peripheral joint of predetermined thickness, thereby forming saidreinforced boat hull shell structure.
 9. A method of forming a shellstructure according to claim 8 wherein:(a) said peripheral member isspaced a preselected distance from inner surfaces of said inner andouter shell moulds with said preselected distance being orthogonal tosaid predetermined distance, (b) providing a flange within saidpreselected distance, extending from one of said shell moulds, with saidflange, peripheral member and peripheral portions of said shell mouldsmeeting at a peripheral juncture, (c) forming a flexible seal at saidperipheral juncture to prevent leakage of said plastic material in saidmolten state at said peripheral member during moulding prior to saidadvancement of said inner and outer shell moulds to close saidperipheral gap, and (d) displacing excess molten plastic material atsaid peripheral member and flange, immediately adjacent said peripheralmember, to form a peripheral joint of a predetermined thickness at saidperipheral juncture.
 10. A method of forming a reinforced boat hullshell structure according to claim 8 wherein said inner shell mould isshaped to provide at least two stiffening member forming surfaces toform at least two longitudinal stiffening members.
 11. A method offorming a reinforced boat hull shell structure according to claim 9,wherein said flange is in the form of an edge which is inclined to faceoutwards.
 12. A method of forming a shell structure using rotarymoulding techniques including the steps of:(a) aligning oppositelydisposed inner and outer shell moulds in a spaced apart relationship,said moulds having inner surfaces, being shaped with complementaryperipheral edges, and one or both of said moulds being shaped to createa stiffening member in a shell structure formed therebetween, (b)providing a peripheral member spaced at a preselected distance from saidinner surfaces of said inner and outer shell moulds, and a flangeextending from one of said shell moulds, (c) forming a flexible seal ata juncture of said peripheral member, said flange, and said peripheraledges of said shell moulds to prevent leakage of plastic material in amolten state adjacent said peripheral member during rotational moulding,(d) rotationally moulding a shell from said plastic material in saidmolten state on said inner surfaces of said oppositely disposed mouldsand said peripheral member, and (e) advancing said inner and outermoulds towards each other while applying said plastic material in saidmolten state to said inner surfaces of said shell moulds to form saidstiffening member, while predeterminedly displacing excess plasticmaterial between said peripheral member and said flange immediatelyadjacent said peripheral member to form a peripheral joint, therebyforming said shell structure.
 13. A method of forming a shell structureaccording to claim 12 wherein said shell structure comprise a hull andsaid oppositely disposed moulds comprise inner and outer shell mouldsfor said hull, and said inner mould is shaped to create at least onelongitudinal stiffening member for said hull, including the steps of:(a)aligning said inner shell mould and said outer shell mould in a spacedapart relationship such that peripheral portions thereof are inalignment, (b) providing said peripheral member around a periphery ofsaid inner and outer shell moulds such that a peripheral gaptherebetween is covered, said peripheral member being arranged such thatsaid oppositely disposed moulds may be advanced towards each other overa predetermined distance, (c) moulding a hull from said plastic materialin said molten state on opposed inner surfaces of said inner and outershell moulds and said peripheral member, and (e) advancing said innerand outer shell moulds towards each other to close said peripheral gapwhilst said plastic material is in said molten state so that contact ismade between said plastic material in said molten state on said innerand outer shell moulds at a location of said stiffening member, suchthat said plastic material in said molten state becomes homogeneous atsuch contact, whilst excess plastic material at said peripheral edges ispredeterminedly displaced towards said inner surfaces of said inner andouter shell moulds at said peripheral member.
 14. A method of forming ashell structure according to claim 12 wherein said inner shell mould isshaped to provide at least two longitudinal stiffening members.
 15. Amethod of forming a reinforced plastic boat hull structure using rotarymoulding techniques, including the steps of:(a) providing a pair ofopposed inner and outer shell moulds, said inner shell mould includinginterior and exterior surfaces, and said inner shell mould including aplurality of stiffening member forming surfaces extending towardscomplementary receiving surfaces at said interior surface of said outershell mould, (b) aligning said pair of moulds in spaced apartrelationship by a predetermined distance, (c) rotationally moulding ashell from plastic material in a molten state on opposed surfaces ofsaid spaced apart inner and outer shell moulds, and (d) advancing saidmoulds towards each other whilst said plastic material is in said moltenstate so that contact is made between said plastic material in saidmolten state on said stiffening member forming surfaces of said innermould and said complementary receiving surfaces of said outer mould,such that said plastic material in said molten state becomes homogeneousat said contact to form stiffening members between said inner shellmould and said interior surface of said outer shell mould, said innershell mould further including surfaces defining spaced webs extendingfrom ends of said stiffening members and defining freely extendingflanges connecting said webs, thereby forming hollow structures betweensaid stiffening members, thereby forming said reinforced plastic boathull structure.
 16. A method of forming a reinforced plastic boat hullstructure according to claim 15, wherein said inner shell mould isshaped to provide a plurality of longitudinal stiffening members onopposite sides of a keel.
 17. A method of forming a reinforced plasticboat hull structure according to claim 15, wherein said pair of shellmoulds are shaped such that said homogeneous plastic material formingportions of said stiffening members at said interior surface of saidouter shell mould is thicker than said plastic material forming saidfreely extending flanges.
 18. A method of forming a reinforced plasticboat hull structure according to claim 15, wherein at least some of saidstiffening member forming surfaces are coincident with a strake formingportion of said outer mould.
 19. A method of forming a reinforced shellstructure using rotary moulding techniques, including the steps of:(a)providing a pair of opposed inner and outer shell moulds, said outershell mould including interior and exterior surfaces, said inner shellmould including a plurality of stiffening member forming extensions,each having a projecting portion which extends towards said outer shellmould and a base forming surface at its terminus which is opposed to acomplementary receiving surface at said interior surface of said outershell mould, (b) aligning said pair of moulds in spaced apartrelationship by a predetermined distance such that there is a gap ofpredetermined extent between said stiffening member base formingsurfaces of said inner mould and opposed complementary receivingsurfaces at said interior surface of said outer mould, (c) rotationallymoulding a shell from plastic material in a molten state of a desiredcoating thickness on opposed surfaces of said spaced apart inner andouter shell moulds, and (d) advancing said moulds towards each otherwhilst said plastic material is in said molten state to close said gapso that contact is made between said plastic material in said moltenstate on said stiffening member base forming surfaces of said innermould and said complementary receiving surfaces above said interiorsurface of said outer mould, such that said plastic material in saidmolten state becomes homogeneous at said contact surfaces to formstiffening members having bases of a predetermined thickness at saidinterior surface of said outer shell mould, said predetermined thicknessbeing dependent upon said coating thickness of said plastic material onsaid inner and outer moulds, thereby forming said reinforced shellstructure, each said stiffening member including integral connectingportions extending from its base towards said inner shell mould alongsaid projecting portion of said stiffening member extension, said innershell mould further defining freely extending flanges connecting ends ofsaid connecting portions, thereby forming hollow structures between saidstiffening member bases.
 20. A method of forming a boat hull shellstructure from oppositely disposed moulds which comprise inner and outershell moulds for said boat hull, including the steps of:(a) aligningsaid inner shell mould and said outer shell mould in a spaced apartrelationship such that peripheral portions thereof are in alignment andspaced apart by a peripheral gap of predetermined distance, (b)providing a peripheral member around a periphery of said inner and outershell moulds such that said peripheral gap therebetween is covered, saidperipheral member being arranged such that said oppositely disposedmoulds may be advanced towards each other over said predetermineddistance, (c) said peripheral member being spaced a preselected distancefrom inner surfaces of said inner and outer shell moulds, with saidpreselected distance being orthogonal to said predetermined distance,(d) providing a flange within said preselected distance, which extendsfrom one of said shell moulds to the other of said shell moulds, (e)forming a flexible seal at a juncture of said flange, said peripheralmember, and said shell moulds, (f) rotationally moulding said boat hullfrom plastic material in a molten state on opposed inner surfaces ofsaid inner and outer shell moulds and on said peripheral member, withsaid flexible seal preventing leakage of said plastic material in saidmolten state at said peripheral member during moulding prior toadvancement of said inner and outer shell moulds to close saidperipheral gap, and (g) advancing said inner and outer shell mouldstowards each other over said predetermined distance to close saidperipheral gap whilst said plastic material is in said molten state,displacing excess plastic material in said molten state at saidperipheral member and said flange, immediately adjacent said peripheralmember, to form a peripheral joint of a predetermined thickness at ajuncture of said inner and outer shell moulds, thereby forming said boathull shell structure.
 21. A method of forming a boat hull structureaccording to claim 20, wherein said flange is in the form of a wedgeintegral with said inner shell mould inclined to face outwards towardssaid peripheral member.